Introduction: Fix a Non-working Electronic Fly Swatter

Picture of Fix a Non-working Electronic Fly Swatter

A friend brought me her new electronic fly swatter she bought in another city. It never worked and taking it back would be too difficult. In addition, her swatter was modelled after a tennis racket. It was not the exact unit shown here. She and her husband really enjoy tennis. So, it was important to make the swatter work for her.

Step 1: Open the Case and Examine the Circuit Board

Picture of Open the Case and Examine the Circuit Board

This is the circuit board from the unit shown in the introductory frame. My friend's circuit board was similar, but not identical.

Beginning from the right side and moving to the left, you see the battery leads from the switches and the red "on" LED. One of the resistors limits the current to the LED. The other provides the correct bias current to the transistor (the black object at the lower right corner of the circuit board). Transformers cannot work with direct current. The transistor switches on and off rapidly to create a rising and falling voltage for the transformer to step up. Next comes the transformer. My friend's unit has only one capacitor for storage of the charge while it builds up to the maximum charge of the capacitor. This unit has two capacitors and four diodes. I did not check it out thoroughly, but I believe the diodes and the capacitors make a voltage multiplier. At the far left of the circuit board are four bleeder resistors to dissipate the charge on the capacitors when one is finished using the swatter. The bare wires are the high voltage wires to the grid wires in the yellow swatter frame.

Step 2: What I Found

Picture of What I Found

The photo shows the transistor. When I opened my friend's swatter, the smooth flat front of the transistor was largely blown away and looked like a miniature bomb crater. That was a good clue for identifying the problem.

Sometimes you can check a transistor while it is in the circuit by using the diode check function on your meter. You are looking for current to flow in one direction between two legs, but not in the other. My friend's transistor blocked all current in both directions. That was further confirmation the transistor was defective.

While this one has numbers on the face that can help to identify a replacement, there were none on my friend's transistor. NPN transistors are more common than PNP. I guessed these circuits use a low voltage NPN switching transistor, like a 2N2222. These are very common and available at Radio Shack on a blister pack.

Step 3: Replace the Transistor

Picture of Replace the Transistor

I did not wick the old solder away from the holes in the circuit board, but melted the old solder with a small wattage soldering iron from below while pushing the leads into the holes from above.

Always use a heat sink when soldering the leads of semiconductors, like transistors. They can easily be ruined by too much heat.

When I finished soldering the transistor leads, I made sure the solder joints looked good. Then I put the circuit board back into the swatter frame and closed it up. (On the one shown here, the battery leads broke and had to be soldered back in place at the battery holder and one of the switches.) When I put the batteries in and pressed the switches, I rested a screwdriver on one of the grid wires and moved it near to the next grid wire. A large spark leaped from the next grid wire to the tip of screwdriver.

The transistor is the most likely component to fail in a circuit like this. Also check any diodes. I believe my friend's new swatter failed when tested at the factory. A high voltage wire was too close to part of the low voltage circuit. High voltage probably flowed back to the transistor and blew it to pieces.

Update: (September 15, 2009) The fly swatter shown in these pictures became weak in its output and then stopped working altogether. The transistor seemed to test good, but when I replaced it the LED lit again. Still, there was no spark when resting a screwdriver on one grid wire and moving it toward the next grid wire. I checked the capacitors and diodes. All seem to be good. I took ohmmeter readings on the transformer windings. The secondary read 268 ohms, but the secondary windings on an identical working unit showed 423 ohms. I am suspicious the secondary winding of the transformer has a short between its windings. Findings a replacement transformer at a reasonable price is not likely. I will probably buy a replacement fly swatter.


kishort (author)2016-07-04

sir if anyone get in touch with secondary side of this circuit, can it cause of high electric shock which can lead to fatality accident???

jerrymonkey123 (author)kishort2016-07-09

actually it is powered by two aa batteries.the current is very small and even the voltage is steped high,the amp will be low and amp is the most danger thing.

Phil B (author)kishort2016-07-04

I have accidentally received a shock from the secondary side of this circuit. I did not enjoy it and plan to avoid it in the future. But, it was not enough to be fatal.

kishort (author)Phil B2016-07-04

thank you sir for good advice!

MinuS5 (author)2016-06-03

fly swatter is working perfectly but it produces noise sometime even if there is no mosquito

Phil B (author)MinuS52016-06-03

A snapping, sparking noise? You might operate it in the dark to see if you can determine where it sparks.

arockia jegadees (author)2015-11-18

My bat was working condition.but it not get mosquito dead sound and not get dead blink light.

Phil B (author)arockia jegadees 2015-11-19

You could have any one of several problems from weak or dead batteries to a broken wire connection to a defective transformer or a failed transistor. If you have the ability, or a friend who does, methodically check the circuit from beginning to end. Otherwise, these bug killers are not expensive and you may simply need to buy a new one.

Chetan KumarS (author)Phil B2015-11-19

HI Phil, Can i use this mosquito circuit output as input to glow 1w LEDs connected in series/parallel.

Phil B (author)Chetan KumarS2015-11-20

I really do not know. I doubt it.

ImdadH (author)2015-10-27


Phil B (author)ImdadH2015-10-27

Thank you for looking and for your comment.

DuartmaN (author)2015-07-14

Hoe can I construct a transformer like that?

Phil B (author)DuartmaN2015-07-15

As someone commented, many of these components are in the flash circuit of disposable cameras. It seems, though, you could get the same effect with two transformers and a diode. Connect the second transformer's secondary to the output of the first transformer. Connect a diode to the primary of the second transformer for the low voltage current to switch the transistor on and off. The second transformer could be a lighter duty, lower wattage transformer.

DuartmaN (author)Phil B2015-07-15

thanks, I will try it :)

harikrishnan.kj.37 (author)2014-12-24

hello again tanx for the explaination.....but now my bat is facing a problem it wont recharge the battery .....i tried using a charged battery ..then it works ...but after the charge gets over i cant recharge it through the circuit.....?

Are you using a battery charger circuit to recharge the battery? It almost sounds as if you want to use the fly swatter to recharge the battery. That old not work. Let me know if I can help you further.


i am trying to use the bat like a normal bat itself......the led wil switch on when ac is supplied but the battery wont charge....i tired using another battery too

harikrishnan.kj.37 (author)2014-12-02

can you please explain the detail .wid a more simple circuit.....i hav the circuit same as that of shown by "yourtubemeera"

I did try to explain it fairly simply. The goal is to raise the voltage in the two small batteries to several hundred volts and use that to charge the wires in the grid so an insect is electrocuted when it short circuits any two wires in the grid.

Capacitors store the high voltage until it is released by the insect completing the circuit. The capacitors are paired with diodes acting like one-way valves. The voltage is able to come into the capacitors, but cannot escape until an insect shorts the wires. You can learn more about this by studying voltage doubler or voltage multiplier circuits.

A transformer raises the voltage of the batteries. But, there is a problem to be overcome. Batteries are DC current and a transformer can work only with a fluctuating current, like AC. The battery current can be made to fluctuate by being shut off and turned back on rapidly. A small special portion of the transformer's secondary winding becomes energized whenever the transformer is turned on. That feeds a small current back to a transistor in the primary circuit of the transformer, and shuts the transistor off, which stops the current flow in the transformer. When there is no feedback current to the transistor to shut it off, the current from the batteries turns it on again and current begins to flow in the transformer again. That causes another feedback current that shuts the transistor off again. All the while bursts of high voltage are moving from the main part of the transformer's secondary winding through the voltage multiplier circuit to be stored in the capacitors.

I hope this helps.

RaviK3 (author)2014-09-10

How much power the output is?.wheather output is ac(or)dc.

Phil B (author)RaviK32014-09-10

It is battery powered, so that part is DC. The transistor causes the DC current to rise and fall in a simulated AC current so the transformer can raise the voltage, but it is still a DC current that is stored in the capacitors. An AC current would only pass through the capacitors and would not build up to make a powerful charge.

lwiker (author)2013-09-28

Hello, when I press the operation button, the red light become dim so fast.
Which component might be failed?

Phil B (author)lwiker2013-09-28

I have only limited experience with electronics. A quick Internet search said your problem could be weak or dirty connections, maybe even in the battery holder. Check the switch contacts, too. Of course, your batteries should be fresh. I suspect something could be providing an alternative pathway for the battery current that takes away the needed current to light the LED properly. That could be a short in the transformer primary windings. An easy way to check would be to borrow an identical electronic flyswatter and take a resistance reading on the primary windings. Then compare that to the same reading you get on your defective flyswatter. A shorted transformer will show a much lower resistance reading. You could also check the actual resistance of the current limiter resistor on the LED and compare it to the resistor's rated value by comparing the colored bands with a table that gives their values so you know the nominal value of the resistor.

This could be a fun mental exercise to see if you can find the problem and fix it. At the same time, a new flyswatter is relatively inexpensive and a person cannot afford to spend too much time fixing one of these, unless there is an unusual reason to invest the time.

denny245 (author)2013-05-17

Weird Coincidence: I just repaired my fathers electronic fly swatter this morning. I took it apart. I cleaned a few bugs that were carbonized and maybe shorting out the high voltage output. Result: Nothing changed. I then used volt ohm meter to check if current flowing through the four diodes and afirmative. Then I measured voltage across the output and it was between 80 and 100 volts but would not pop when shorted by my screw driver. It sparked alittle but not the way it was supposed. I then suspected the 30 volt storage capacitor was somehow burned out. Without removing the capacitor I connected a 300 volt capacitor from a Xenon flash circuit, and it quickly charged up and when shorted out with the screwdriver, an explosion of sparks shot out with a loud pop sound. That was the problem. I replaced the storage capacitor with one rated at 100 volts, and now my fathers Fly Zapper tennis racket works again!!!

Phil B (author)denny2452013-05-18

Excellent. Thank you for adding to the body of knowledge. It will be useful to someone.

TeslaRox (author)2013-02-01

Just built my first Tesla Coil using the board from the ol faithful Electric Flyswatter, worked fantasic! The problem was I shortened my primary coil wire and that improved it more, better arcs and the spark gap was a bit more lively. The down side of that was after around 45 seconds its power reduced drasticly.. I also hooked up another 2 AA's in seires with the main 2. It still makes the tiniest of sparks but not sufficient as it did when it was still a swatter. I am not the best with electronic componants myself but am learning fast. Can anyone suggest what this issue might be or how I could mod the exsisting board to handle and release more power. OR.. perhaps even an alternative power source that operates the same as the swatter, thanx.

tesla man (author)2012-06-28

In your instructable, you said that the transistor switches on and off rapidly. How would you go about duplicating this process??(in other words, how does it work?)

Phil B (author)tesla man2012-06-28

When the flyswatter is switched "on" current flows through the biasing resistor to turn the transistor "on" and energize the transformer primary. The rise in energy in the transformer primary induces a voltage in the transformer secondary. But, a steady voltage in the primary will not induce a voltage in the secondary for long, only while the voltage in the primary is either rising or falling rapidly. The transformer also has a loop in it that feeds back to the transistor and over-biases the transistor so that it shuts "off." When the transistor is "off," the current that over-biased it ceases because the current going to the primary goes through the transistor, and then the battery current through the transistor's biasing resistor turns the transistor back "on" so current again flows in the transformer primary. It is a simple and cheap oscillator.

If you want to duplicate something like this, get two floodlights controlled by photocells so each shuts off when the sun rises. Point each at the photocell of the other. They will come on and go off in a continuous cycle.

I hope this helps.

tesla man (author)Phil B2012-06-29

Yes, it helped, but could you give me a schematic of how to make the biasing resistor circut. I would like to know because i am trying to power a transformer with dc. Thank you in advanced.

Phil B (author)tesla man2012-06-29

Take a look at my response to mattccc below. There you will find a link to a schematic for the electronic flash in a disposable camera. It is quite similar to the circuit in an electronic fly swatter.

tesla man (author)Phil B2012-06-30

Ok thank you

NukeWeldor (author)2011-11-28

This is some great information, and timely too, since I just replaced my original Electrocution swat with a new one that is a real dud. You can barely hear it buzz when you activate the circuit. Couldn't kill a fly with it if you tried and was completely useless on other pests like lady bugs, honey bees or butterflys.

With this help I'm sure to be back at the top of my game again in no time.

Thanks so much... .

Phil B (author)NukeWeldor2011-11-29

Thank you for looking and for commenting. I hope you can get yours working properly. It is always possible the transformer has an internal short that keeps it from developing full power. Someone suggested taking parts from the flash unit on a disposable flash camera.

-max- (author)2010-07-04

take apart the flash circuit in a disposable camera there shold be a transformer in it.

Phil B (author)-max-2010-07-06

That is an excellent idea. I do not work with or use disposable cameras. I am guessing the physical size of the transformer in a camera is quite a bit smaller than the transformer in an electronic fly swatter, and that might affect the jolt delivered to an insect. (Larger transformer = more output) I just do not know.

Alex1M6 (author)Phil B2010-10-27

I know this sounds like more trouble than its worth but take a look inside of a CCFL inverter. There you will find an adequate transformer for your bug zapper.
(I know as I have once used one to drive a home-made high voltage transformer/stun gun.

-max- (author)Phil B2010-07-08

not really unless your talking about watts. (watts = volts X amps) the voltage should be about the same (400 volts).

joinaqd (author)2009-01-22

what kind of transistor must it be for the transformer to work?i got transistors from a CPU Cooler fan..

askiwn (author)joinaqd2010-07-25

bd243c 2sd965 work good

Phil B (author)joinaqd2009-01-23

If you already have some transistors you want to use in an application like repairing a fly swatter, you will need to know if they are PNP or NPN. My Instructable on Test a Bi-Polar Transistor--Out of Circuit will help you. When the transistor comes "on," check to see what polarity you had to use on the connections to the meter.

Phil B (author)joinaqd2009-01-22

See Step 2. I guessed that it was a low voltage NPN switching transistor of type 2N22222 and my guess was correct. The circuit worked after replacing the transistor with one of this type.

yourtubemeera (author)2010-07-21

hello phil B i also have that swatter...its not working...but i cannot find any burnt or damaged component inside...but ya the transister in it becomes very hot when its in operation... the swatter is not giving enuff voltage to kill the mosquitoes.. wat shall i do...?? here is the cicuit... its from YPD company....china made...

Phil B (author)yourtubemeera2010-07-22

If a component becomes hot, it usually means it is carrying far more current than it should. That would lead me to suspect there is a short circuit in the primary windings of the transformer. The fact your unit does not put out the amount of voltage it should tends to confirm that. Check it by taking resistance readings (ohms) between transformer terminals and compare those with the same readings taken on the transformer of an identical unit. Look for one of the readings on your unit to be quite a bit lower than on the known good unit. Then consider Powermax's suggestion about the transformer from the flash in a disposable camera, or get a new fly swatter.

yourtubemeera (author)2010-07-21

the images of my swatter circuit

mattccc (author)2010-06-25

will you post a schmatic of the circuit please thank you

Phil B (author)mattccc2010-07-06

Somehow I did not see your request until now. The schematic is very similar to the electronic flash circuit in a disposable camera. In place of the xenon tube insert grid wires to be bridged by an insect. This link discusses the circuits on a disposable camera flash unit.  Scroll down the page for a schematic.

Phil B (author)2010-05-26

This is advertising.  I reported it to the moderators.  The poster put the same thing on several other sites.  I thought it might be removed by now.

pskog (author)2009-03-05

Want to boost? Add additional high voltage capacitor parallel with the last big one from where connecting wire will go to the grids. This will give you more sparks to nail those nasty bugs.

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Bio: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying ... More »
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